1. Field of Invention
The field of the present invention relates in general to modems and more particularly to a method and apparatus for a variable bandwidth X-DSL modem.
2. Description of the Related Art
North American Integrated Service Digital Network (ISDN) Standard, defined by the American National Standard Institute (ANSI), regulates the protocol of information transmissions over telephone lines. In particular, the ISDN standard regulates the rate at which information can be transmitted and in what format. ISDN allows full duplex digital transmission of two 64 kilo bit per second data channels. These data rates may easily be achieved over the trunk lines, which connect the telephone companies' central offices. The problem lies in passing these signals across the subscriber line between the central office and the business or residential user. These lines were originally constructed to handle voice traffic in the narrow band between 300 Hz to 3000 Hz at bandwidths equivalent to several kilo baud.
Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, VDSL, HDSL all of which are broadly identified as X-DSL have been developed to increase the effective bandwidth of existing subscriber line connections, without requiring the installation of new fiber optic cable. An X-DSL modem operates at frequencies higher than the voice band frequencies, thus an X-DSL modem may operate simultaneously with a voice band modem or a telephone conversation. Currently there are over ten discrete X-DSL standards, including: G.Lite, ADSL, VDSL, HDSL2, SHDSL, and other DSLs all of which are broadly identified as X-DSL.
Currently there are over ten discrete XDSL standards, including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc. Within each standard there are at least two possible line codes, or modulation protocols, discrete multi-tone (DMT) and carrierless AM/PM (CAP). A typical DMT system utilizes a transmitter inverse discrete Fourier transform (IDFT) and a receiver discrete Fourier transform (DFT). The following patents are related to DMT modems: U.S. Pat. No. 5,400,322 relates to bit allocation in the multi-carrier channels; U.S. Pat. No. 5,479,447 relates to bandwidth optimization; U.S. Pat. No. 5,317,596 relates to echo cancellation; and U.S. Pat. No. 5,285,474 relates to equalizers. The following patents are related to CAP modems: U.S. Pat. No. 4,944,492 relates to multidimensional pass band transmission; U.S. Pat. No. 4,682,358 relates to echo cancellation; and U.S. Pat. No. 5,052,000 relates to equalizers. Each of these patents is incorporated by reference as if fully set forth herein.
XDSL modems are typically installed in pairs, with one of the modems installed in a home or business and the other in the telephone companies central office (CO) switching office servicing that home. This provides a direct dedicated connection to the home or office from a line card at the central office on which the modem is implemented through the subscriber line or local loop.
Each installation represents a sizeable expense in hardware and service labor to provision the central office. The expense may not always be amortized over a sufficient period of time due the relentless introduction of new and faster xDSL standards each of which pushes the performance boundaries of the subscriber line in the direction of increasing bandwidth and signal integrity. As each new standard involves, line cards must typically be replaced to upgrade the service. Not all subscriber lines qualify for the higher bandwidths offered by the evolving X-DSL protocols. Subscriber line length, i.e. the distance from the home or business to the central office is one of the primary factors determining the ability of a subscriber line to support higher data rates. Evolving standards like VDSL call for data rates from 3.75 Mega Bits per second (Mbps) up to 13 Mbps and higher. Typically only a small percentage of the installed subscriber lines with loop distances less than 2000 feet from the central office will qualify for the upper data rates, the higher bandwidths. The hardware and processing capability needed to deliver high bandwidths to these short loops is expensive, and can not be utilized on the longer subscriber loops.
What is needed is a less rigid signal processing architecture that supports scalability of CO resources, and allows a more flexible hardware response to the evolving XDSL standards and the problems associated with providing hardware to handle each new standard.